N-methylspiperone Binding Research Articles

Effects of Endogenous Neurotransmitters on the in vivo Binding of Dopamine and 5-HT Radiotracers in Mice

Several studies have indicated that the in vivo binding of D2 receptor positron emission tomography radiotracers can, under some conditions, be influenced by competition with endogenous dopamine. The present study was undertaken to compare the extent to which the in vivo binding in mice of radiotracers to other amine neuroreceptors, namely D1, 5-HT2A and 5-HT1A receptors, can also be modulated by neurotransmitter competition. For dopamine radiotracers we examined [3H]raclopride as a D2 radiotracer and [3H]A69024 as a D1 radiotracer. Striatal binding of both radiotracers was substantially reduced by administration of the dopamine releaser, amphetamine, although only at a high dose. [3H]raclopride was decreased more than [3H]A69024. Dopamine depletion with 4-hydroxybutyrate strongly increased [3H]raclopride binding but failed to increase [3H]A69024 binding. For 5-HT radiotracers we examined [3H]N-methylspiperone as a 5-HT2A radiotracer and [3H]WAY 100635 as a 5-HT1A radiotracer. Cortical binding of both radiotracers was unaffected by the 5-HT releaser, p-chloroamphetamine. [3H]WAY 100635 binding was additionally unaffected by 5-HT release with fenfluramine and by 5-HT depletion with p-chlorophenylalanine.In conclusion, of the four radiotracers examined, [3H]raclopride binding to D2 receptors had greatest sensitivity to changes in endogenous neurotransmitter levels. [3H]A69024 binding to D1 receptors was affected only by neurotransmitter increases. [3H]N-methylspiperone binding to 5-HT2A receptors and [3H]WAY 100635 binding to 5-HT1A receptors appeared insensitive to changes in neurotransmitter levels.

A consideration of the dopamine D2 receptor monomer-dimer equilibrium and the anomalous binding properties of the dopamine D2 receptor ligand, N-methyl spiperone.

Some discrepancies between experimental results with the two D2 antagonists N-methyl spiperone (NMSP) and raclopride (RAC) have been observed. Among these are the observation that MK-801 increases NMSP binding but not RAC binding: pretreatment with reserpine increases RAC binding but decreases NMSP binding; and that the two ligands yield different values for Bmax. It has been observed that the D2 receptor can exist in both a monomer and dimer form and that a NMSP photolabel ligand binds primarily to the monomer form while a RAC-like photolabel ligand binds both. Using a model of the dimerization in which the equilibrium dissociation constant increases with increasing dopamine (DA) concentration, the free monomer concentration can be shown to go through a maximum value with increasing DA. Using this model with data from a baboon PET study, it can be shown that under certain conditions an increase in binding could be observed. Further research may show that there are clusters of D2 receptors forming oligomers with more than two receptors in which NMSP binds to more sites on clusters with fewer receptors. If increasing DA favors cluster with fewer receptors, an increase in NMSP binding sites may also occur under some circumstances with an increase in DA.

Serotonin 5-HT 2 receptors in schizophrenic patients studied by positron emission tomography

Using positron emission tomography (PET) and [ 11C]N-methylspiperone (NMSP), we examined 5-HT 2 receptors in the cortex of schizophrenic patients in whom we previously observed decereased prefrontal D1 receptor binding. The subjects were 10 neuroleptic-naive schizophrenic patients, 7 schizophrenic patients who were drug-free but had previously been treated with neuroleptics, and 12 normal controls. A non-significant trend towards decreased prefrontal [ 11C]NMSP binding was observed in the neuroleptic-treated patients, suggesting a possible effect of previous neuroleptic treatment on the alteration in cortical 5-HT 2 function. However, the neuroleptic-naive patients showed no noticeable difference in cortical [ 11C]NMSP binding compared to controls. Our results do not rule out the role of 5-HT 2 function as a crucial site of therapeutic activity of schizophrenia, but they do suggest that cortical 5-HT 2 receptors might not be primarily involved in the pathophysiology of schizophrenia.

Changes in apparent in vivo binding of [3H]raclopride and [3H]N-methylspiperone induced by oxotremorine.

The effects of muscarinic agonist, oxotremorine (0.3 mg/kg), and antagonist, scopolamine (0.5 mg/kg), on in vivo [3H]raclopride (RAC) and [3H]N-methylspiperone (NMSP) binding were investigated. Following tracer administration to control or pretreated mice, binding potentials, and the rate constants k3 and k4 were determined by kinetic analysis. Oxotremorine resulted in a 70% increase in striatal RAC binding potential compared with controls. RAC and NMSP showed almost identical decreases in k3 (40%), whereas k4 for RAC was unexpectedly decreased by 64%. Scopolamine resulted in no significant changes in RAC or NMSP binding. These results, in combination with previous data obtained in reserpinized mice, show that 1) competition by endogenous ligand may not be the only factor influencing the magnitude of apparent in vivo receptor binding, and 2) interneuronal communication may be partly mediated by changes in the rates of ligand-receptor binding.

Opposing effects of clomipramine on [125I]RTI-55 and [3H]N-methylspiperone binding in mouse striatum: important role of other factors than endogenous dopamine?

Opposing effects of clomipramine on [125I]RTI-55 and [3H]N-methylspiperone binding in mouse striatum: important role of other factors than endogenous dopamine?

Sustained Withdrawal Allows Normalization of In Vivo [11C]N-Methylspiperone Dopamine D2 Receptor Binding after Chronic Binge Cocaine: A Positron Emission Tomography Study in Rats

In our previous positron emission tomography studies striatal binding for both [11C]SCH23390 and [11C]N-methylspiperone (NMSP) were decreased in the rat brain on the last day of chronic (14 days) binge cocaine administration. We have found that [11C]SCH23390 binding to dopamine D1 receptors returns to saline control levels within ten days withdrawal from chronic binge cocaine and remains at control levels after 21 days withdrawal. An 18% decrease in [11C]NMSP binding to dopamine D2 receptors was observed after ten days withdrawal. However, importantly, after 21 days withdrawal [11C]NMSP binding was at saline control levels. Changes of in vivo [11C]NMSP binding required a longer abstinence period for normalization than [11C]SCH23390 binding. The apparent recovery of dopamine D2 receptors after prolonged abstinence from chronic cocaine and the different rates of normalization for dopamine D1 versus D2 receptors may be critical information for development of pharmacotherapies for cocaine dependent patients.

Use of positron emission tomography to measure the effects of nalmefene on D1 and D2 dopamine receptors in rat brain

Positron emission tomography (PET) has been used in humans and in non-human primates to image and measure radioligand binding to neuroreceptors. The present study evaluated the feasibility of performing high-resolution PET experiments in a rodent model to measure receptor kinetics. The effects of acute and chronic administration of the opioid antagonist, nalmefene, on the binding activity of [11C]SCH23390 and [11C]N-methylspiperone at D1 and D2 dopamine receptors, respectively, was investigated in the rat. The interaction between central opioid and dopaminergic systems has been the focus of much attention due to their interactive role in mediating reinforcement and locomotor activity. In the present study, adult male Sprague–Dawley rats received either a single injection of 10 mg/kg of nalmefene or control vehicle solution 1 h prior to the PET scan or were chronically administered 10 mg/kg/day of nalmefene or vehicle for 7 days by an osmotic minipump. Following acute administration of nalmefene, the binding potential of [11C]SCH23390 in the striatum was significantly increased. No changes in [11C]N-methylspiperone binding were found. Following chronic nalmefene administration, no significant change in either [11C]SCH23390 binding potential or [11C]N-methylspiperone binding was detected. These results suggest that nalmefene administration produces transient changes in the binding potential of D1-receptors in the striatum that are normalized after 1 week of steady-state administration.

Reserpine-induced reduction of in vivo binding of SCH 23390 and N-methylspiperone and its reveral by d-amphetamine

In order to clarify the role of endogenous dopamine in the binding of [ 3H]Isch223390 and [ 3H]N-methylspiperone to the mouse striatum in vivo, the effects of reserpine and the reversal of these effects by d-amphetamine were investigated. Radioactivity was measured in the striatum and cerebellum following i.v. injection of each ligand into control and drug-treated mice. The ratio of radioactivity in the striatum to that in the cerebellum, plotted as a function of time, showed a linear correlation. Pretreatment with reserpine 24 h prior to injection of tracer significantly decreased the in vivo binding of both [ 3H]SCH23390 and [ 3H]N-methylspiperone in a dose-dependent manner. Saturation experiments indicated that these changes in in vivo binding were due mainly to changes in apparent affinity rather than to the number of binding sites available. Administration of d-amphetamine to reserpine-treated mice reversed the effect of reserpine in a dose-dependent manner. Blockade of dopamine D; receptors with SCH23390 did not prevent the reversal by d-amphetamine of 3H]N-methylspiperone binding in vivo; however, treatment with haloperidol did prevent the effect of d-amphetamine on [ 3H]SCH23390 binding. These results suggest that dopamine D; receptor-mediated neurotransmission might itself regulate the binding of dopamine to receptors in vivo.

Difference in in vivo receptor binding between [3 H]N-methylspiperone and [3 H]raclopride in reserpine-treated mouse brain

The in vivo binding of [3H]N-methylspiperone (NMSP) and [3H]raclopride was compared in mice treated with reserpine (5 mg/kg, 24 hr prior to the tracer injection). With both radioligands, selective accumulation of radioactivity in the striatum following intravenous injection was observed, whereas a relatively low accumulation and a rapid decline in radioactivity in the cerebellum was seen. Reserpine significantly decreased [3H]NMSP binding in vivo, however it increased [3H]raclopride binding. By compartment model analysis, it was found that the decrease in [3H]NMSP binding was primarily due to the decrease in the association rate (K3) and the increase in [3H]raclopride was due to the decrease in the dissociation rate (K4) in vivo. As both Kd and Bmax of dopamine D2 receptors have been reported to be unaltered by reserpine, these results suggested that some unknown factors except Kd and Bmax which influence on in vivo binding of receptors might be changed by reserpine. These results revealed that it is of importance to measure kinetics of ligand-receptor binding in vivo rather than static analysis. These two different types of radioligands can be combined to reveal functional roles of dopamine receptor in vivo, especially in the study of the human brain with positron emission tomography (PET).

Asymmetry in D-2 binding in female rat striata

An asymmetry in D-2 receptor densities (B max) measured by [ 3H]N-methyl spiperone binding was found in striata from female Sprague-Dawley rats. The D-2 B max on the right side was on average 40% greater than the D-2 B max on the left side. This asymmetry is greater in magnitude and opposite in direction to that reported for males and is independent of directional preference exhibited during nocturnal circling.